This invention relates to connectorizing and tuning polarization maintaining (PM) optical fibers.
In optical fiber communications, connectors for joining fiber segments at their ends, or for connecting optical fiber cables to active or passive devices, are an essential component of virtually any optical fiber system. The connector or connectors, in joining fiber ends, for example, has, as its primary function, the maintenance of the ends in a butting relationship such that the core of one of the fibers is axially aligned with the core of the other fiber so as to maximize light transmissions from one fiber to the other, or, put another way, to reduce insertion loss. Another goal is to minimize back reflections. Alignment of these small diameter fibers is extremely difficult to achieve, which is understandable when it is recognized that the mode field diameter MFR of, for example, a singlemode fiber is approximately nine (9) microns (0.009 mm). The MFR is slightly larger than the core diameter. Good alignment (low insertion loss) of the fiber ends is a function of the transverse offset, angular alignment, the width of the gap (if any) between the fiber ends, and the surface condition of the fiber ends, all of which, in turn, are inherent in the particular connector design. The connector must also provide stability and junction protection and thus it must minimize thermal and mechanical movement effects.
In the present day state of the art, there are numerous, different, connector designs in use for achieving low insertion loss and stability. In most of these designs, a pair of ferrules (one in each connector), each containing an optical fiber end, are butted together end to end and light travels across the junction. Zero insertion loss requires that the fibers in the ferrules be exactly aligned, a condition that, given the necessity of manufacturing tolerances and cost considerations, is virtually impossible to achieve, except by fortuitous accident. As a consequence, most connectors are designed to achieve a useful, preferably predictable, degree of alignment, some misalignment being acceptable.
However, in connecting or terminating polarization maintaining (PM) fibers, such is not the case. Many optical fiber components, such as, for example, interferometers and sensors, lasers, and electro-optic modulators, are extremely sensitive to and dependent upon, for proper operation, the polarization of the light. Even very slight alterations or changes in the light polarization orientation can result in wide swings in the accuracy of response of such devices. PM fiber has polarization-dependent refractive indices, and the speed of light in an optical fiber is inversely proportional to the magnitude of the refractive index. A birefringent optical fiber is one having two polarizations having different velocities of propagation, thus giving rise to a xe2x80x9cfastxe2x80x9d wave and a xe2x80x9cslowxe2x80x9d wave. In a PM fiber, the polarization of a linearly polarized light wave input to the fiber, with the direction of polarization parallel to that of the one of the two principal polarizations, will remain or be maintained in that polarization as it propagates along the fiber, hence the term xe2x80x9cpolarization maintaining.xe2x80x9d If the polarization of the light wave is to be maintained at a splice or other connection, the principal axes of birefringence of the two joined fibers must be aligned in parallel, otherwise there will be polarization cross-coupling, i.e., crosstalk, which is highly undesirable. Thus, where two PM fibers, for example, are to be connected together, they should be terminated carefully to reduce the crosstalk during the connectorization process. Also, the connectors must be capable of aligning then maintaining the fiber orientation to the connector key position. Connectors with tolerances adequate for connecting non-PM fibers usually are inadequate for maintaining polarization alignment at the connector junction.
Typical PM connector requirements are an insertion loss of less than 0.3 dB, and the prior art PM connector arrangements comprise numerous, different connector configurations aimed at meeting these requirements for different connectors, such as an SC type connector as shown in U.S. Pat. No. 5,216,733 of Ryo Nagase et al. The connector of that patent comprises a ferrule body and a ring shaped flange having a keyway mounted on the periphery of the ferrule body. Alignment is achieved by rotating the ferrule body with respect to the flange keyway. The combination of ferrule and flange comprises a plug which is inserted into a push-pull SC connector having a key therein for mating with the flange keyway and springs bias the flange in the longitudinal direction to maintain the alignment.
In U.S. Pat. No. 4,784,458 of Horowitz, a splice joint for PM fibers is shown wherein aligned fibers are joined with UV curing epoxy, and the joint is overlaid with epoxy cement for rigidity. Such a joint is permanent, and does not function as a connect-disconnect optical fiber connector.
U.S. Pat. No. 5,561,726 of Yao discloses an apparatus for controlling the polarization state of the light within a fiber by squeezing a portion of the fiber to produce a birefringent fiber, and the squeezer is then rotated to change the polarization of the light within the fiber. The device is not a connector, but is intended for use with polarization sensitive devices such as interferometers and electro-optic modulators, however, it may also be used with connectors for connecting two PM fibers.
It is common practice in the prior art for creating PM fibers to include a pair of rods in the fiber cladding which extend parallel to the core as shown in U.S. Pat. No. 4,515,436 of Howard et al. Such rods, which are preferably of glass, are, in manufacture of the fiber, included in the fiber preform from which the fiber is drawn. As the fiber is drawn, the rods are accordingly diminished in diameter and are located within the cladding, preferably on either side of the core. The rods have different thermal expansion characteristics than the surrounding glass, and the stress they exert on the core causes the index of refraction to change along that axis. The axes then have different indices of refraction value and thus propagate light at different speeds. Variations on the two rod arrangement are also known, such as the elliptical stress member disclosed in U.S. Pat. No. 5,488,683 of Michal et al. Also, squeezing the fiber to create birefringence, as shown in the aforementioned Yao patent is feasible. The two rod PM fiber, so called xe2x80x9cPandaxe2x80x9d type PM fiber, however, has proven quite satisfactory in use, and it is toward the connectorization of such a fiber that the present invention is directed, although other types of PM fibers may be used with the present invention.
In the copending U.S. patent application Ser. No. 10/151,613 of Lampert, et al. and U.S. Pat. No. 6,619,856 of Lampert, et al, are shown, respectively, a PM connector plug and an adapter therefor the principles of which are applicable to any of a large number of optical fiber connectors, but are embodied in a modified LC connector in those applications. For optimum performance, i.e., maximum transmission of a polarized beam, it is highly desirable to provide accurate rotational positioning of better than xc2x11xc2x0 or even as accurate a  less than xc2xcxc2x0 between connectors equipped with polarization maintaining fibers.
The present invention is an apparatus and method for tuning the PM connectors of those applications to achieve this desideratum.
When a PM jumper cable, for example, is terminated by connectors, it is most desirable that the cable/connector combination be tuned to align the fiber slow axis with the connector key which serves as a reference point. In accordance with the present invention, there is provided a tuning apparatus for performing the tuning method of the invention which yields extremely accurate rotational positioning of the connectors.
The apparatus, which is similar to that shown in TIA/ETA Standard FOTP-193, comprises a first assembly including a coupling stage comprising a light source, and a polarizer interposed between first and second connector adapters and connector plugs. A second assembly having a second coupling stage, spaced from the first coupling stage comprises a connector adapter (the second coupling stage), a power meter, the output of which is connected to a PC, and another polarizer (or analyzer). Both polarizer and analyzer can be rotated to any angle and controlled by a rotation controller connected to the PC. In use, a jumper cable, for example, terminated by connector plugs, is inserted into the adapters in the first and second coupling stages, and the polarizer in the first stage is rotated to match the slow polarization axis of the connector, determined by the increased power reading. Linear polarized light is then launched into the slow axis of the PM fiber. The analyzer in the second stage is rotated and the output power varies between maximum and minimum, as indicated by the power meter. As will be discussed hereinafter, the crosstalk in dB is calculated as the difference between maximum and minimum power.
The tuning of the PM connector is to set the PM fiber slow axis to correspond to the key position of the connector, which preferably is the connector latch or latching arm. Therefore, two joined PM jumpers can have the same slow axis alignment according to key position to minimize crosstalk due to misalignment. Once the polarization direction of the analyzer is aligned to the connector key which can be regarded as a master position, the tuning process can easily be done by matching the fiber slow axis to the analyzer direction as indicated by output light power. In order to align the analyzer to the key or master position, a pair of PM jumper cables are connected to each other and to the first and second stages, and the crosstalk of the connection is then measured, and one connector is tuned for the lowest crosstalk in the connection of two jumpers.
One of the jumpers is then moved and the other is connected between the first and second stages. The angle of the maximum output is defined as zero degrees. By measuring the crosstalk, the analyzer can be aligned to the slow axis of the fiber and the analyzer position is recorded. The one jumper is then removed and the second is connected between the stages and the analyzer is also aligned to the second jumper""s slow axis which generally will occur at a different angle than that of the one jumper.
The position of the key will be midway between the two angles and the analyzer is rotated to the master position and thus the ferrules of the connectors are rotated to this value, which is designated as zero. At this point the slow wave orientation of the connectors is parallel to the connector key, and the connector of the PM jumpers are optimally lined. With the master analyzer position thus determined, subsequent alignment of connectors becomes a single rotation of the ferrules to conform.
With the connector plug and adapter of the aforementioned Lampert, et al applications, the ferrules of the connector plug are maintained, with very slight possibility of variation, in the optimum tuned position.